This invention relates to a method of producing acrylamide polymers, more specifically, this invention relates to an improved method for polymerizing concentrated aqueous monomer solution. Obtained acrylamide polymers are water soluble, porous, hydrous gel and can be dried quickly.
Various processes for preparing polymers of acrylamide have been already known in the art, and many products of acrylamide polymers are now available on the market. The techniques on production of such material are also rapidly advancing. The commercially feasible acrylamide polymers may be grouped into two types: aqueous liquid and powder. These two types of polymers have their own traits, and both of them are widely used in many fields of industry. Recently, however, preference is being given to use of the powder as it has now become possible to obtain the powder with very high quality and also because such powder are more easy to treat.
Various kinds of techniques have been proposed for producing powdery acrylamide polymers, for instance, reversed phase emulsion polymerization method, reversed phase suspension polymerization method and precipitation polymerization method. Any of these methods, however, must use an organic solvent in the polymerization process and also can not always produce a polymer with high quality. There is also available a method in which an aqueous monomer solution at a relatively low concentration (15 to 20% by weight) and dehydration is made by using a water-absorbing solvent such as methanol, acetone or acetonitrile. According to this method, however, a great quantity of water-absorbing solvent must be used as water exists in an amount of more than 5 times the polymer, and also too much cost is required for recovery of the solvent.
Japanese Patent Laid-Open No. 124188/74 discloses "A method of producing dry acrylamide polymers" according to which 10 to 30% by weight of aqueous acrylamide solution is polymerized, and the obtained polymerization product is kneaded and shaped by using an extruder, followed by primary drying with hot air and secondary drying with methanol. This method requires a great quantity of heat as the initial water content is high.
According to the "Method of producing water-soluble polymers" proposed in Japanese Patent Publication No. 5222/74, high-concentration solution of acrylamide (50 to 80% by weight) is polymerized in the presence of a polymerization regulator by forming the polymerization solution into the form of a thin film to increase the surface area while removing heat of polymerization by means of radiation or external cooling. This method has a drawback that the polymerization vessel used therefor is excessively large in area because the polymerization solution is formed into a film, and hence such method is also not suited for industrial application.
In the past attempts for improvement in polymerization of acrylamide, priority has been given to the techniques for removing heat of polymerization and various methods have been proposed in this connection. For instance, in the reversed phase emulsion polymerization method or reversed phase suspension polymerization method, it is tried to dissipate heat of polymerization in a great amount of organic solvent introduced into the polymerization system. In the case of aqueous solution polymerization, as it is difficult to remove heat of polymerization, the monomer concentration usable in this method is up to about 30% by weight, and for polymerization at a higher concentration, the polymerization solution must be formed into a film, so as to expedite heat dissipation. Thus, the greatest problem in the conventional art of aqueous solution polymerization was how to keep the polymerization system below the boiling point to prevent the thermal degradation of the polymer.